Dental scalers remove plaque from teeth by vibrations, which cause tartar to be dislodged from the teeth and the spaces between the teeth. Various types of powered dental scalers are known, the most common of which are the electrical ultrasonic transducer and the air driven sonic vibrating models. Ultrasonic scalers require complex electronic components for controlling the frequency of vibrations and are therefore quite expensive. Air driven sonic scalers are often subject to high frequency shaft vibration, which makes them subject to wear in the shaft-rotor area, thereby necessitating frequent servicing and replacement of parts.
Some models of currently available hand-held dental scalers, such as the Titan.RTM. SW model sonic scaler, manufactured by Star Dental Products, Lancaster, Penn., remove plaque from teeth by vibrations generated by the rotation of an air-driven rotor around a shaft. In these models the rotor oscillates against the surface of the shaft, thereby creating vibrations which initiate the self resonant frequency of the shaft that in turn excites the tip of the scaler to vibrate at the same frequency. These scalers are preferred over other scalers because of their ease of use, efficiency and enhanced durability.
U. S. Pat. No. Re. 29,687, issued to Sertich, the disclosure of which is incorporated herein by reference, describes a scaler in which vibrations are generated by the rotation of a sleeve-shaped rotor around a hollow tubular shaft. The shaft has two or more rows of passages which extend radially outward from the hollow interior of the shaft to its external cylindrical surface. The rotor, which has an inside diameter slightly greater than the outside diameter of the shaft, is maintained in position over the external compressed air ports of the passages by means of air flow equilibrium. Guide rings are provided as stops for control at start-up and until the rotor stabilizes. The guide rings are mounted on the tubular shaft, one on each side of the rows of ports, and they are spaced sufficiently to permit the rotor to move back and forth slightly in opposite axial directions during rotor start-up or run-down conditions. The rotor of the Sertich scaler is rotated by directing air through the central opening of the shaft and out through the radial passages. The air is discharged from the ports in a direction somewhat tangential to the outer surface of the shaft and after it leaves the ports it impinges on the inside surface of the rotor, thereby causing the rotor to rotate at high speeds, setting the shaft into resonant vibration in a range of 5000 to 6000 Hertz. The high rate of rotation of the rotor, together with its axial and radial oscillation during rotation, causes the shaft to vibrate at about 6,000 Hertz. The vibration set up in the shaft is transmitted to the tip of the scaler.
The shafts of air-driven dental scalers having shaft-mounted sleeve rotors are desirably made of a special brass alloy because of its superior resonance properties and ease of machinability. Unfortunately, brass is very soft and cannot withstand the significant frictional forces and impacts imparted to the shaft during operation of the scaler. To eliminate this problem, it is the practice, when constructing the rotor shafts out of brass, to plate a very thin layer of a more durable metal onto the outer surface of the shaft in the rotor area. The more durable metal acts as a bearing surface for the rotor. A preferred plating metal is nickel, which, in addition to its durability, provides a smooth, corrosion resistant surface.
The current procedure for finishing dental scaler rotor shafts is to plate an alloy containing about 90% nickel and 10% phosphorus onto the surface of the shaft. It is usually necessary to further enhance the bearing properties of nickel-phosphorus alloy plated rotor shafts. One method of accomplishing this is to pit the sidewall of the plated shaft and impregnate the pitted surface with polytetrafluoroethylene, e.g. Teflon.RTM.. The Teflon.RTM. provides a smooth bearing surface on the shaft, thereby reducing the tendency of the shaft to undergo excessive wear.
A number of disadvantages attend the use of nickel-phosphorus alloy as a plating material for dental scaler rotors. One disadvantage is that the plated part must be heat-treated at a temperature of about 400.degree. C. for an hour or more to harden the alloy, thus further increasing the complexity and cost of the shaft manufacturing process. A second and more serious disadvantage is that the nickel-phosphorus alloy tends to soften upon being repeatedly subjected to elevated temperatures, which, of necessity, occurs after each use because the scaler must be sterilized after use at a temperature of up to about 135.degree. C. for an autoclave or chemiclave sterilizer. In a dry heat sterilizer, the scaler is subjected to temperatures up to 175.degree. C. After repeated sterilizations, the nickel-phosphorus layer softens and the shaft plating wears rapidly, thereby substantially shortening its useful life. A third disadvantage is the lack of uniformity of the finished shafts. It is difficult to control accurately the degree of pitting, and the amount of Teflon.RTM. impregnated into the surface of the shaft can vary widely from shaft to shaft. The lack of uniformity results in an inordinately high percentage of shaft rejects.
Because of the above-noted disadvantages there is a continuing effort to develop dental scaler rotor shafts which can be more simply manufactured and which have superior durability and bearing properties and which is not affected adversely by heat sterilization temperatures.